AGN predictions for the Hubble Deep Field and the X-ray background

We estimate the number of AGN among the galaxies in the Hubble Deep Field(HDF). The recent discovery of a class of X-ray luminous narrow emission-linegalaxies has provided a possible explanation for the origin of the X-raybackground (XRB), although the nature of the activity is still unresolved. Byextrapolating the observed X-ray number count distribution to faint opticalmagnitudes we predict that this AGN population could account for a significantfraction, $\sim 10$ per cent, of the galaxies in the HDF. In contrast, normalbroad line QSOs are expected to account for no more than $\sim 0.1$ per cent ofthe sources at these magnitudes.